I have disconnected the cooler pipes at the front end and tried pushing oil back though both pipes and they are both very restricted

That's VERY strange. Have the pipes been damaged by sharp bends or tight tie wraps or clamps?

Whatever the inside diameter, there should be no restriction.

Doug.

Don't forget about correct installation of the oil cooler banjo bolt on the engine that has a check ball inside of it. (From reading this thread though, I don't believe that is your issue, but be aware of it.) This banjo bolt with the check ball should be installed on the transmission side of the engine case. The other banjo bolt does not have a check ball. If they are reversed, bad things happen.

There are no kinks or damage to the steel pipes, the one way bolt is in the correct place, I had a thought last night to try and eliminate different parts so I by-passed the cooler at the front and we are back to 20psi on start up and no rise in pressure when the engine speed increases. 

Would someone with a newer Z8 measure the outside diameter of the steel pipes for me please (mines a 2015)

I've made a decision and if the oil stays cool enough, I'm going to leave the cooler to another time, I have very good oil pressure now, 70 psi hot at 6200 rpm

[2015 Z Force 800 53/Trail EPS]

It's pretty clear there is something wrong, and whatever that is had the the oil pump and relief maxed out which will kill it and the pump soon...not to mention the reduced flow will allow increased heat. You know it's in that system somewhere and I think you are right in it being better off without the plugged system over better pressure and flow. I think I would go a head and get a universal cooler and line kit for it for now though and maybe mount it in the rear for now somewhere just to take the edge off.

Over the years of being a motorcycle mechanic I've seen lots of lines and passages blocked by all kinds of things that surprise me on how they got there but..have not seen anything large enough get in a cooler line to effect it's flow...yet. Maybe this is the first..and maybe whatever it is has always been there from the DOM and the root cause of all your engine problems. In fact..I'd put money on it.

I agree there must have been a lot of stress on those plastic gears etc, I have only run her for a few seconds each time when the oil cooler was connected, I also think the problem has been there since new.

I have thought about running new lines to the existing cooler, but I think I will check the old lines when I can to see what is up with them.

My rush to get her running Is because I have bought a snow plough and we don't get much snow so I want to make the most of it with my boys if we have any.

I've also been out on a test run for about 6 miles and the by pass hose is cool enough to hold so I'm guessing it's not overheating the oil, will do a better test when I find my infra red thermometer

[2015 Z Force 800 53/Trail EPS]

On my Brute I would run it for a few hours pretty hard then stop, pull the dip-stick and drop my probe from my Fluke meter into the oil for a reading. My friend would do the same but with a simple stick gauge.

Been out for a 20 mile drive and checked the crankcase temp which was 55 deg C, I understand the oil normally runs a similar temp to the water so I'm thinking it will be fine over winter without the oil cooler

55 C=131 F. At that temperature, the oil is far from being worked hard. In a typical engine that's warmed up, oil temp should be around 10 to 15 F degrees warmer than the coolant temperature. And as long as your oil temp doesn't exceed 275 F under constant load you should be fine, but also guard against colder than normal oil too due to condensates that can collect in the crankcase.

[2015 Z Force 800 53/Trail EPS]

Yeah 131F isn't even close to operating temp. Oil needs to be above 165F preferably above 180F. Next time I'm out I am going to get some readings on mine. 

Sounds like the oil cooler is over kill when the weather is colder in the winter months, a water cooled heat exchanger may be better as it will warm the oil quicker and keep it at a more workable temp,  other than that a oil cooler thermostat may be a good idea